This invention relates to the manufacture of metal bead rings for pneumatic tires and especially to bead rings comprising filaform elements helically wound or wrapped about a circular core member for use in radial tires.
In the manufacture of pneumatic tires, a metal bead ring is generally applied to opposite ends of a number of superposed plies of rubber-coated fabric wrapped around a cylindrical tire building drum. The ends of the fabric plies are turned up over the rings which are ultimately embedded in rubber between the plies to form the beads of the vulcanized pneumatic tire. While many types of both multi-filament and mono-filament bead rings have been used, current practices are believed to favor multi-filament beads due to their superior strength and flexibility.
It is desirable that the filament have a helical wrap about the circular axis of symmetry defined by the cross section of the bead in order to provide for some resilient radial expansion of the bead ring as well as for other resilient flexing. The cross sectional form of the bead is also an important factor, a fairly smooth form being desirable in order to reduce the possibility of tearing or cutting of the plies by sharp edges of the bead. A better securing of the bead to the rubber is achieved where the cross section of the bead is irregular rather than completely round. Accordingly, it is desirable to have a bead formed of filaments helically wrapped about a circular axis of symmetry and circumferentially spaced apart to define helical grooves therebetween for anchoring the rubber plies to the bead ring. Furthermore, it is desirable that the wire filaments be of long lengths sufficient for several convolutions, rather than of short lengths extending only one convolution and requiring the connecting together of the ends of each length for each complete loop. It is also desirable that the assembled bead as viewed in cross section have adjacent convolutions of wire tangent to one another as well as to the core ring.
One such type of helically wrapped multi-filament bead ring utilizes a circular core ring about which the outer filaments are helically wrapped while another type is of coreless construction and formed, for example, in the manner described in U.S. Pat. No. 3,251,384. The present invention is directed to the manufacture of helically-wrapped, multifilament, solid-core-type bead rings.
Machines of the prior art for forming bead rings, grommets and the like of helically wrapped wire, such as the machines described in U.S. Pat. Nos. 1,294,160, 1,327,170 and 1,414,828 of Pratt, generally pass a lead length of wire through a circular path to form a ring and then continue to revolve the ring about its axis of symmetry. The supply reel or spool feeding the wire is orbited around and through the ring in correspondence with the speed of rotation of the ring about its axis of symmetry. The revolving ring, at it is formed, pulls the wire from the reel and the ring is progressively enlarged until its formation is complete.
The mechanism for rotating the ring varies and can comprise stationary driver rollers, an endless chain with an arcuate span, an endless belt, etc. It is, of course, esssential that the ring run through an open or free unsupported span or gap wherein the wire being supplied may be laid onto the ring. The wire being fed from the reel is often fed through a die that orbits with the spool and which is adapted to form a helical twist in the wire to facilitate its wrapping.
Most of the prior art machines, however, are not adapted to form helical convolutions of wire around a prefabricated core ring--especially a core ring of a substantially larger diameter than the wire. None of the prior art machines are capable of placing each successive helical convolution of wire at a uniform circumferential spacing from the next adjacent convolution on the core ring and none place the wire so accurately relative to the core ring that the leading end of the first laid convolution registers accurately with the trailing end of the last laid convolution.
A particular problem encountered in the past with machines of the type described is that as the wire is fed onto the core ring or the like from an orbiting reel, an axial twist is produced in the wire (as opposed to a helical twist about the circular axis of the core ring as defined by the centroid of the ring's cross section) thus causing the wire to have a tendency to unwind.
Another problem is that in the formation of a bead of the type described above, where a relatively thin wire is helically wrapped progressively around a core ring of relatively large diameter, the helices must be laid and spaced tangent to one another around the circumference of the core ring cross section and the number of the helices in each convolution must be very accurately controlled so that the ends of the wire at the first and last convolution may be connected to one another.
The method and apparatus of the present invention, reduce the difficulties inherent in the prior art and afford other features and advantages heretofore not obtainable.